This invention relates generally to motor controllers and more particularly to a power factor type motor controller for AC induction motors.
The induction motor is one of the most commonly used motors. It operates at a constant speed which is independent of both load and applied voltage within certain limits. As pointed out in U.S. Pat. No. 4,404,511 issued Sep. 13, 1983 to Frank J. Nola and assigned to the National Aeronautics and Space Administration, an induction motor will consume much more energy than it requires for operation when it is operating under light load conditions. The voltage applied to an induction motor should be a function of the load for efficient operation. Because of the variations in the operating line voltage, most induction motors are designed to deliver rated load plus a safety margin at an undervoltage which is less than the typical line voltage rating. By way of example, a 115-volt motor may be designed to deliver its rated load at 105 volts. In addition, because induction motors draw the same current whether loaded or unloaded, the motor efficiency goes down when less than a rated load is applied to the motor. In other words, an AC induction motor will consume much more energy than it requires under light load conditions.
In the prior motor controller design of Nola, a resistor is used to sense the motor inrush current. The circuitry is used to generate a control signal which represents the phase shift difference between the motor current and the motor voltage, i.e., the power factor. As loading on the motor changes the power factor will change and the off time of the duty cycle of the AC line power applied to the motor is changed. As the power factor decreases, the duty cycle is decreased, i.e., the motor is turned off for a longer period of time. In this arrangement, the amount of power savings is scaled according to the power sensed.
My prior patent application titled Microcomputer Controlled Load Controller U.S. Ser. No. 08/134,207, which was filed Oct. 8, 1993, describes a microprocessor based load controller. However, like the Nola circuit, the illustrative embodiment of my prior circuit utilized a toroidal core transformer to provide current sampling.
A.C. induction motors are rather inefficient when they are not matched properly to their load. A motor is most efficient when it is heavily loaded and the rotor "slips" from its unloaded synchronous speed. As a bi-product of rotor slip, power factor is effected making the current closer in phase to the voltage.
Rotor slip can be induced not only by loading a motor, but also by reducing its average applied power in such a manner as to remove sections of the sinusoidal excitation waveform. Such a technique can greatly enhance the efficiency of the motor under light load conditions. The ultimate results is realized in power savings and lower operating costs.
It is one object of the present inventor to provide an arrangement in which the toroidal core transformer is eliminated.